Why can't I change the subscripts in a molecule when I balance a chemical reaction?

1 Answer
Truong-Son N.
Dec 5, 2016

Because you change the integrity of the compound you're looking at.

For example:

#6"HCl" + 2"Al"(s) -> 2"AlCl"_3 + 3"H"_2(g)#

If you suddenly write #6"HCl"# as #3"H"_2"Cl"_2#, then what you've done is replace #"HCl"# with a new molecule whose formula is #"H"_2"Cl"_2#, and this molecule may or may not exist. This one doesn't.

The new chemical formula of #"H"_2"Cl"_2# literally tells you that there are two hydrogen atoms and two chlorine atoms in this hypothetical molecule, but #"HCl"# only had one of each.

Likewise, #3"H"_2"Cl"_2# is also not the same as #3"H"_2 + 3"Cl"_2#.

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